Only update image parent href in if it matches the expected value in PlaceholdersPostFilter

See here:

https://gitlab.com/gitlab-org/gitlab/-/blob/1301f88369b5f65f668733fcb8e8da95781de094/lib/banzai/filter/placeholders_post_filter.rb#L355

Changing the href of an anchor can have security-relevant implications, since the ReferenceRedactor relies on it. Verify the target <a> has the expected href (corresponding with the <img> we're changing it from), since while we expect these to be produced by ourselves, there's nothing stopping a user entering this HTML manually.